Steel for low-temperature service having excellent surface processing quality and method for manufacturing same

ABSTRACT

Provided is a steel sheet for low-temperature service, which can be used at a wide temperature range from low temperature to room temperature in liquefied gas storage tanks and transport facilities. The steel sheet for low-temperature service has an excellent surface processing quality even after a processing processes is performed, such as a tension process. The a steel sheet contains manganese (Mn, 15-35 wt %), carbon (C, satisfying 23.6C+Mn≧28 and 33.5C—Mn≦23), copper (Cu, 5 wt % or less (excluding 0 wt %)), chrome (Cr, satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %)), titanium (Ti, 0.01-0.5 wt %), nitrogen (N, 0.003-0.2 wt %), the balance iron (Fe), and other inevitable impurities. Ti and N satisfy relational expression 1 below. [Relational expression 1] 1.0≦Ti/N≦4.5 (Mn, C, Cr, Ti, and N in the respective expressions mean wt % of respective ingredient contents).

TECHNICAL FIELD

The present disclosure relates to steel for low temperature environmentshaving excellent surface processing qualities and a method ofmanufacturing the same.

BACKGROUND ART

Steel used for storage containers containing liquefied natural gas,liquid nitrogen, or the like, and used for offshore platforms andfacilities in polar regions may be provided as steel for low temperatureenvironments maintaining sufficient toughness and strength even atextremely low temperatures. Such steel for low temperature environmentsshould have excellent low-temperature toughness, strength, and magneticproperties, as well as having relatively low coefficients of thermalexpansion and thermal conductivity.

Recently, steel (Patent Document 1) having excellent extreme lowtemperature properties through the addition of relatively large amountsof manganese (Mn) and carbon (C), with nickel (Ni) completely excluded,to stabilize austenite and including aluminum (Al) have been used. Inaddition, steel (Patent Document 2) having excellent low-temperaturetoughness in such a manner that a mixed structure of austenite andepsilon martensite is secured by adding Mn thereto has been used.

In the case of steel for low temperature environments having austeniteas a main microstructure thereof, relatively large amounts of C and Mnare added thereto, thereby stabilizing austenite. However, an additionof C and Mn affects the recrystallization behavior of austenite, therebycausing partial recrystallization and nonuniform grain growth in arolling temperature range of the related art. Thus, only a specificsmall number of austenite grains are significantly grown, therebycausing significant nonuniformity in the size of austenite grains in amicrostructure.

In general, in the case of austenite structures having relatively highcontents of C and Mn, deformation behavior is implemented by slips andtwin crystals in a manner different from general carbon steel. Inaddition, in the early stage of deformation, deformation behavior isusually implemented by slips corresponding to uniform deformation, buttwin crystals corresponding to nonuniform deformation are subsequentlyaccompanied thereby. When the size of grains is relatively large, stressrequired to form twin crystals is reduced, thereby easily generatingtwin crystals even in the case of a relatively low degree ofdeformation. In a case in which a relatively small number of coarsegrains are present in a microstructure, deformation of twin crystalsoccurs in coarse grains in the early stage of deformation, therebycausing nonuniform deformation. Thus, surface characteristics ofmaterials may be deteriorated, thereby causing nonuniform thicknesses offinal structures. In detail, in the case of structures requiringinternal pressure resistance by securing uniform thicknesses of steel,such as low-temperature pressure vessels, significant problems instructural design and use thereof occur.

Thus, in the case of steel, a microstructure of which has beenaustenitized by adding C and Mn thereto, steel for extreme lowtemperature environments, produced at low cost, which is economical andhas secured structural stability by improving the uneven surfaces causedby early deformation of coarse grains into twin crystals is urgentlyrequired to be developed.

PRIOR ART DOCUMENT

-   Patent Document 1: Korean Patent Application No. 1991-0012277-   Patent Document 2: Japanese Patent Application No. 2007-126715

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide steel for lowtemperature environments having excellent surface processing qualitiesand a method of manufacturing the same.

Technical Solution

According to an aspect of the present disclosure, steel for lowtemperature environments having excellent surface processing qualitiesincludes 15 wt % to 35 wt % of manganese (Mn), carbon (C) satisfying23.6C+Mn≧28 and 33.5C—Mn≦23, 5 wt % or lower of copper (Cu) (excluding 0wt %), chrome (Cr) satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %), 0.01 wt% to 0.5 wt % of titanium (Ti), 0.003 wt % to 0.2 wt % of nitrogen (N),iron (Fe) as a residual component, and inevitable impurities. Ti and Nsatisfy Relational Formula 1 below.

According to an aspect of the present disclosure, a method ofmanufacturing steel for low temperature environments having excellentsurface processing qualities includes providing a slab including 15 wt %to 35 wt % of Mn, C satisfying 23.6C+Mn≧28 and 33.5C—Mn≦23, 5 wt % orlower of Cu (excluding 0 wt %), Cr satisfying 28.5C+4.4Cr≦57 (excluding0 wt %), 0.01 wt % to 0.5 wt % of Ti, 0.003 wt % to 0.2 wt % of N, Fe asa residual component, and inevitable impurities, Ti and N satisfyingRelational Formula 1 below; heating the slab at a temperature of 1050°C. to 1250° C.; and manufacturing heat-rolled steel by heat rolling theslab that has been heated.

1.0≦Ti/N≦4.5,  [Relational Formula 1]

where Mn, C, Cr, Ti, and N in each expression refer to wt % of a contentof each component.

In addition, the foregoing technical solution does not list an entiretyof characteristics of the present disclosure. Various characteristics ofthe present disclosure and consequent advantages and effects will beunderstood in more detail with reference to specific exemplaryembodiments below.

Advantageous Effects

According to an aspect of the present disclosure, steel for lowtemperature environments having excellent surface processing qualitieseven after being processed due to an austenite structure having uniformparticle sizes and a method of manufacturing the same may be provided.

DESCRIPTION OF DRAWINGS

FIG. 1A is an image captured using an optical microscope, illustrating amicrostructure of steel for low temperature environments of the relatedart.

FIG. 1B is an image of a cross section of a specimen after steel for lowtemperature environments of the related art is tensioned.

FIG. 2 is an image captured using an optical microscope, illustrating amicrostructure of steel for low temperature environments according to anexemplary embodiment in the present disclosure.

FIG. 3 is a graph illustrating ranges of carbon (C) and manganese (Mn)controlled in an exemplary embodiment.

BEST MODE FOR INVENTION

The inventors recognized that, in the case of steel having an austenitestructure, containing a relatively large amount of carbon (C) andmanganese (Mn), partial recrystallization and grain growth of theaustenite structure occurs in a rolling temperature range of the relatedart, thereby generating abnormally coarse austenite; in general,critical stress required to form a twin crystal is higher than that of aslip, but in a case in which a size of a grain is relatively great forthe reason described above, stress required to form the twin crystal isreduced, thereby causing deformation of the twin crystal in the earlystate of deformation, so that a problem in which surface quality may bedegraded due to discontinuous deformation may occur. In addition, theinventors have conducted in-depth research to solve the problemdescribed above.

Thus, the inventors confirmed that steel for low temperatureenvironments in which fine austenite is uniformly distributed may beobtained in such a manner that a titanium (Ti)-based precipitate isproperly educed by adding Ti thereto, in order to suppress significantcoarsening of an austenite grain and realized the present disclosure.

Hereinafter, steel for low temperature environments having excellentsurface processing qualities according to an exemplary embodiment willbe described in detail.

According to an aspect of the present disclosure, the steel for lowtemperature environments having excellent surface processing qualitiesincludes 15 wt % to 35 wt % of manganese (Mn), carbon (C) satisfying23.6C+Mn≧28 and 33.5C—Mn≦23, 5 wt % or lower of copper (Cu) (excluding 0wt %), chrome (Cr) satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %), 0.01 wt% to 0.5 wt % of titanium (Ti), 0.003 wt % to 0.2 wt % of nitrogen (N),iron (Fe) as a residual component, and inevitable impurities. Inaddition, Ti and N satisfy Relational Formula 1 below.

1.0≦Ti/N≦4.5,  [Relational Formula 1]

where Mn, C, Cr, Ti, and N in each expression refer to wt % of a contentof each component.

First, an alloy composition of the steel for low temperatureenvironments having excellent surface processing qualities according toan exemplary embodiment will be described in detail. Hereinafter, a unitof each alloying element is wt %.

Manganese (Mn): 15% to 35%

Mn is an element playing a role in stabilizing austenite in an exemplaryembodiment. 15% or more of Mn may be contained to stabilize an austenitephase at extremely low temperatures. In other words, in a case in whichan Mn content is lower than 15%, when a C content is relatively low,metastable phase epsilon martensite is formed and easily transformedinto α-martensite by strain induced transformation at extremely lowtemperatures, thereby not securing toughness. In a case in which the Ccontent is increased to stabilize austenite to prevent the casedescribed above, physical properties thereof may be dramaticallydegraded due to carbide precipitation. Thus, the Mn content may behigher than or equal to 15%. On the other hand, in a case in which theMn content is higher than 35%, a problem in which a corrosion rate ofsteel is increased, and economic feasibility is reduced due to anincrease in the Mn content occurs. Thus, the Mn content may be limitedto a range of 15% to 35%.

Carbon (C): 23.6C+Mn≧28 and 33.5C—Mn≦23

C is an element stabilizing austenite and increasing strength. Indetail, C plays a role in reducing M_(s) and M_(d), transformationpoints in which austenite is transformed into epsilon martensite orα-martensite by a cooling process or a process. Thus, in a case in whichC is insufficiently added, stability of austenite is insufficient,thereby not obtaining stable austenite at extremely low temperatures. Inaddition, external stress causes strain induced transformation in whichaustenite is easily transformed into epsilon martensite or α-martensite,and toughness and the strength of steel is reduced. On the other hand,in a case in which the C content is significantly high, toughness isdramatically degraded due to carbide precipitation, and workability isdegraded due to a significant increase in strength.

In detail, the C content in an exemplary embodiment may be decided inconsideration of a relationship between C and other elements addedthereto. To this end, a relationship between C and Mn in forming acarbide that the inventor has discovered is illustrated in FIG. 3. Asillustrated in FIG. 3, the carbide is formed using C. C does notindependently affect formation of the carbide, but affects a tendency toform the carbide in combination with Mn.

FIG. 3 illustrates a proper C content. In order to prevent the carbidefrom being generated, on a premise that other components satisfy a rangemade in an exemplary embodiment, a value of 23.6C+Mn (in the case of Cand Mn, a content of each component is expressed using wt %) may becontrolled to be higher than or equal to 28. The value refers to aleftward diagonal line in a hexagonal area in FIG. 3. In a case in which23.6C+Mn is lower than 28, stability of austenite is decreased, andstrain induced transformation is generated by impacts at extremely lowtemperatures, thereby degrading impact toughness. In a case in which theC content is significantly high, that is, 33.5C-Mn is higher than 23, anaddition of a significant amount of C causes carbide precipitation,thereby degrading low-temperature impact toughness. In conclusion, C maybe added to satisfy an entirety of Mn: 15% to 35%, 23.6C+Mn≧28, and33.5C—Mn≦23. As illustrated in FIG. 3, a lowermost limit of the Ccontent is 0%, within a range satisfying the expression above.

Copper (Cu): 5% or lower (excluding 0%)

Cu has significantly low solid solubility in the carbide and isrelatively slow in spreading in austenite, thereby being concentrated inaustenite and at an interface of a nucleated carbide. Thus, spreading ofC is interrupted, thereby effectively slowing carbide growth. As aresult, a generation of the carbide is suppressed. In addition, Custabilizes austenite to improve extreme low-temperature toughness.However, in a case in which a Cu content is higher than 5%, hotworkability of steel is degraded. Thus, an uppermost limit may belimited to 5%. In addition, the Cu content to obtain an effect ofsuppressing the carbide as described above may be higher than or equalto 0.5%.

Chrome (Cr): 28.5C+4.4Cr≦57 (excluding 0%)

Cr plays a role in improving impact toughness at low temperatures bystabilizing austenite and increasing the strength of steel through beingsolubilized in austenite within a range of a proper content thereof. Inaddition, Cr is an element improving corrosion resistance of steel.However, Cr is a carbide element. In detail, Cr is also an elementforming the carbide in an austenite grain boundary to reduce the impactof low-temperatures. Thus, a Cr content in an exemplary embodiment maybe determined in consideration of the relationship between C and otherelements added thereto. In order to prevent the carbide from beinggenerated, on a premise that other components satisfy a range made in anexemplary embodiment, a value of 28.5C+4.4Cr (in the case of C and Cr, acontent of each component is expressed using wt %) may be controlled tobe lower than or equal to 57. In a case in which the value of28.5C+4.4Cr is higher than 57, the generation of the carbide in theaustenite grain boundary is difficult to suppress effectively, due tosignificant contents of Cr and C, thereby causing a problem in whichimpact toughness at low temperatures is degraded. Thus, Cr may be addedto satisfy 28.5C+4.4Cr≦57 in an exemplary embodiment.

Titanium (Ti): 0.01% to 0.5%

Ti is an element forming a TiN precipitate in combination with nitrogen(N). In an exemplary embodiment, during high-temperature hot rolling, aportion of the austenite grain may be significantly coarse. Thus, growthof the austenite grain may be suppressed by properly educing TiN. Tothis end, at least 0.01% or more of Ti is required to be added. However,in a case in which a Ti content is higher than 0.5%, an effect of growthof the austenite grain may not be improved anymore. In addition, coarseTiN is educed, thereby reducing an effect of growth of the austenitegrain. Thus, in an exemplary embodiment, the Ti content may be limitedto a range of 0.01% to 0.5%.

Nitrogen (N): 0.003% to 0.2 wt %

In an exemplary embodiment, in order to effectively achieve a goal ofadding Ti described above, N is required to be added simultaneously. Indetail, in order to effectively educe TiN, 0.003% or more of N may beadded. However, since solid solubility of N is lower than or equal to0.2%, an addition of 0.2% or greater of N is significantly difficult,and 0.2% or less thereof is sufficient to educe TiN, thereby limiting anuppermost limit thereof to 0.2%. Thus, an N content may be limited to arange of 0.003% to 0.2% in an exemplary embodiment.

A residual component of an exemplary embodiment is Fe. However, since,in a manufacturing process of the related art, unintentional impuritiesmay be inevitably mixed from a raw material or a surroundingenvironment, unintentional impurities are unavoidable. Since theimpurities are known to those skilled in the manufacturing process ofthe related art, descriptions thereof will not be provided in detail inan exemplary embodiment.

In addition, a weight ratio of Ti to N, that is, Ti/N, may satisfyRelational Formula 1 below.

1.0≦Ti/N≦4.5  [Relational Formula 1]

In a case in which a Ti/N ratio is controlled to be higher than or equalto 1.0, solute Ti is combined with N, thereby educing minute TiN. Inaddition, since TiN that has been educed using a method described aboveis stably present, the growth of the austenite grain may be effectivelysuppressed.

However, in a case in which the Ti/N ratio is higher than 4.5, coarseTiN is crystallized in molten steel, thereby adversely affecting aproperty of steel and not obtaining uniform distribution of TiN. Inaddition, surplus Ti that has not been educed to be TiN is present in astate of solid solution, thereby adversely affecting heat-affected zonetoughness. However, in a case in which the Ti/N ratio is lower than 1.0,an amount of solute N in a base metal is increased, thereby adverselyaffecting heat-affected zone toughness. Thus, the Ti/N ratio may becontrolled to be 1.0 to 4.5.

In addition, the steel for low temperature environments according to anexemplary embodiment described above may include the TiN precipitatehaving a size of 0.01 μm to 0.3 μm.

In a case in which a size of the TiN precipitate is less than 0.01 μm,the TiN precipitate is easily solubilized, so that an effect ofsuppressing grain growth becomes insufficient. On the other hand, in acase in which the size of the TiN precipitate is greater than 0.3 μm, anaustenite grain pinning effect is reduced, and a coarse size thereofadversely affects toughness. Thus, the size of the TiN precipitate maybe within a range of 0.01 μm to 0.3 μm.

In addition, the steel for low temperature environments according to anexemplary embodiment may include the TiN precipitate in an amount of1.0×10⁷ to 1.0×10¹⁰ per 1 mm².

In a case in which the TiN precipitate is present in an amount less than1.0×10⁷ per 1 mm², a grain pinning effect is insignificant, thereby noteffectively suppressing growth of a coarse grain. On the other hand, ina case in which the TiN precipitate is present in an amount greater than1.0×10⁷ per 1 mm², the size of the TiN precipitate becomes relativelysmall, so that the TiN precipitate may be unstable, and impact toughnessof a material thereof may be degraded. Thus, the amount of the TiNprecipitate may be 1.0×10⁷ to 1.0×10¹⁰ per 1 mm².

In addition, the steel for low temperature environments according to anexemplary embodiment limits the number of coarse austenite grains havinga size of 200 μm or greater in the microstructure to 5 or less per 1cm².

Since, in the case of austenite having a grain size less than 200 μm,stress required to generate the twin crystal is sufficiently higher thanstress required to generate a slip, nonuniform transformation is notgenerated within a transformation rate of steel for low temperatureenvironments of the related art when a structure is manufactured. Thus,the size thereof may be limited to 200 μm or greater. In addition, in acase in which the density of a grain having a size of 200 μm or greateris greater than 5 per 1 cm², due to a relatively high density of thecoarse grain, nonuniform transformation is sufficiently deteriorated toaffect surface qualities. Thus, the density of the grain having a sizeof 200 μm or greater may be limited to 5 or less per 1 cm².

In the meantime, the steel for low temperature environments according toan exemplary embodiment may include an austenite structure in an areafraction of 95% or higher. Austenite, a representative soft structure inwhich ductile fracture is generated even at low temperatures, is anessential microstructure to secure low-temperature toughness and shouldbe included in an area fraction of 95% or higher. In a case in whichaustenite is included in an area fraction of lower than 95%, austeniteis not sufficient to secure low-temperature toughness, that is, impacttoughness of 41 J or greater at a temperature of −196° C., so that alowermost limit thereof may be limited to 95%.

In addition, the carbide present in the austenite grain boundary may belower than or equal to 5% in an area fraction. In an exemplaryembodiment, the carbide is a representative structure that may bepresent, beside austenite. The carbide is educed in an austenite grainboundary and becomes a cause of grain boundary rupture, therebydegrading low-temperature toughness and ductility. Thus, an uppermostlimit thereof may be limited to 5%.

Hereinafter, a method of manufacturing the steel for low temperatureenvironments having excellent surface processing qualities according toanother exemplary embodiment will be described in detail.

The method of manufacturing the steel for low temperature environmentshaving excellent surface processing qualities according to anotherexemplary embodiment includes providing a slab satisfying the alloycomposition described above, heating the slab at a temperature of 1050°C. to 1250° C., and manufacturing hot-rolled steel by hot rolling theslab that has been heated.

Providing a Slab

The slab satisfying the alloy composition described above is provided. Areason for controlling the alloy composition is the same as describedabove.

Heating a Slab

The slab is heated at the temperature of 1050° C. to 1250° C.

A process described above is performed for the sake of solution andhomogenization of a cast structure, segregation, and secondary phasesgenerated in a process of manufacturing the slab. In a case in which thetemperature is lower than 1050° C., homogenization thereof isinsufficient or a temperature of a heating furnace is significantly low,thereby causing a problem in which deformation resistance is increasedduring heat rolling. In a case in which the temperature is higher than1250° C., partial melting may occur and surface qualities may bedegraded in segregation in the cast structure, and TiN may becrystallized, thereby not contributing to austenite refinement, butdegrading properties thereof. Thus, a heating temperature of the slabmay be in a range of 1050° C. to 1250° C.

Manufacturing Hot-Rolled Steel

The slab that has been heated is heat rolled, thereby manufacturing thehot-rolled steel.

In an exemplary embodiment, the alloy composition and the heatingtemperature of the slab, described above, may be satisfied, therebymanufacturing the steel for low temperature environments havingexcellent surface processing qualities. Thus, in detail, it is notnecessary to control a condition of the manufacturing hot-rolled steeland the manufacturing hot-rolled steel may be performed using a generalmethod.

INDUSTRIAL APPLICABILITY

Hereinafter, the present disclosure will be described in more detailthrough exemplary embodiments. However, an exemplary embodiment below isintended to describe the present disclosure in more detail throughillustration thereof, but not limit the scope of rights of the presentdisclosure, because the scope of rights thereof is determined by thecontents written in the appended claims and can be reasonably inferredtherefrom.

After a slab satisfying a component system stated in Table 1 below ismanufactured in the same manner as a manufacturing condition stated inTable 2, a microstructure, yield strength, an elongation rate, Charpyimpact toughness at a temperature of −196° C., or the like, are measuredto be stated in Table 2 or Table 3, respectively.

In Table 3 below, unevenness of surfaces is assessed by observingsurfaces of the steel for low temperature environments with the nakedeye.

TABLE 1 Weight Ratio of 23.6C + 33.5C − 28.5C + Classification C Mn CuCr N Ti Ti/N Mn Mn 4.4Cr Comparative 0.62 18.12 0.12 0.2 0.012 32.8 2.718.6 Example 1 Comparative 0.37 25.4 1.12 3.85 0.018 34.1 −13.0 27.5Example 2 Comparative 0.61 18.13 1.5 1.25 0.012 32.5 2.3 22.9 Example 3Comparative 0.31 28.7 0.15 1.32 0.025 0.024 0.96 36.0 −18.3 14.6 Example4 Comparative 0.45 11.7 0.008 0.07 8.75 22.3 3.4 12.8 Example 5Comparative 0.37 24.1 1.02 3.5 0.011 0.05 4.55 32.8 −11.7 25.9 Example 6Inventive 0.58 21.7 0.61 0.55 0.053 0.06 1.13 35.388 −2.3 19.0 Example 1Inventive 0.45 24.3 0.43 3.08 0.12 0.17 1.42 34.92 −9.2 26.4 Example 2Inventive 0.39 28.6 0.85 3.45 0.016 0.02 1.25 37.804 −15.5 26.3 Example3 Inventive 0.44 27.5 0.42 1.62 0.024 0.04 1.67 37.884 −12.8 19.7Example 4 Inventive 1.1 23.4 1.05 0.87 0.021 0.05 2.38 49.36 13.5 35.2Example 5

In Table 1 above, a unit of a content of each element is wt %.

TABLE 2 Density of Temperature Coarse of Grain of 200 μm HeatingAustenite Carbide TiN or Furnace Fraction Fraction Size No. of TiNgreater Classification (° C.) (%) (%) (μm) (No./mm²) (No./cm²)Comparative 1195 99.1 0.9 10 Example 1 Comparative 1180 99.6 0.4 7Example 2 Comparative 1200 99 1 8 Example 3 Comparative 1195 98.9 0.80.003 1.2 × 10⁴ 7 Example 4 Comparative 1200 82 1 1.25 4.32 × 10⁵  7Example 5 Comparative 1195 99.6 0 0.95 5.2 × 10⁶ 9 Example 6 Inventive1205 99.1 0.8 0.013 5.3 × 10⁸ 0 Example 1 Inventive 1190 99.3 0 0.0154.2 × 10⁸ 0 Example 2 Inventive 1195 99.4 0 0.022 2.9 × 10⁸ 1 Example 3Inventive 1198 99.6 0 0.01 5.4 × 10⁸ 0 Example 4 Inventive 1203 98.7 0.80.025 2.7 × 10⁸ 0 Example 5

TABLE 3 Base Metal Yield Tensile Impact Classifica- Strength StrengthElongation Value Unevenness tion (MPa) (MPa) Rate (%) (J, −196° C.) ofSurfaces Comparative 363 1011 69 83 Occurred Example 1 Comparative 470931 46 130 Occurred Example 2 Comparative 405 1006 57 81 OccurredExample 3 Comparative 411 912 57 130 Occurred Example 4 Comparative 346762 12 38 Occurred Example 5 Comparative 360 926 54 35 Occurred Example6 Inventive 425 980 67 153 Not Example 1 Occurred Inventive 453 902 58148 Not Example 2 Occurred Inventive 468 975 61 165 Not Example 3Occurred Inventive 427 980 65 152 Not Example 4 Occurred Inventive 481971 51 118 Not Example 5 Occurred

In Inventive Examples 1 to 5, it can be confirmed that a componentsystem and a composition range controlled in an exemplary embodiment aresatisfied, and high-quality steel for low temperature environmentswithout uneven surfaces may be obtained in such a manner that a densityof a coarse austenite grain is controlled to be 5 or less per 1 cm² byminute eduction of TiN, and Inventive Examples 1 to 5 are processed. Inaddition, stable austenite in which fraction of austenite in themicrostructure is controlled to be 95% or higher, and fraction of thecarbide is controlled to be lower than 5% may be obtained, therebysecuring excellent toughness at extremely low temperatures.

On the other hand, in Comparative Examples 1 to 3, it can be confirmedthat TiN may not be educed, since Ti is not added thereto, therebygenerating a coarse grain and unevenness of surfaces after ComparativeExamples 1 to 3 are processed.

In detail, in the case of Comparative Example 4, it can be confirmedthat, since the component system and the composition range controlled inan exemplary embodiment are not satisfied, ferrite is generated, therebysignificantly degrading impact toughness. In addition, it can beconfirmed that, since a size and the number of TiN controlled in anexemplary embodiment are not satisfied, the number of coarse grains isincreased, thereby generating unevenness of surfaces.

In addition, in the case of Comparative Examples 5 to 6, it can beconfirmed that Ti and N within a range controlled in an exemplaryembodiment are added, but the weight ratio of Ti to N and a size and thenumber of the TiN precipitate do not satisfy the range controlled in anexemplary embodiment, so that coarse TiN is educed, and the coarse grainis significantly generated to generate unevenness of surfaces afterComparative Examples 5 to 6 are processed.

FIG. 1A is an image of the microstructure of steel of the related art inwhich a nonideal coarse grain is formed by coarsening of the austenitegrain. FIG. 1B is an image of unevenness occurring on a surface of steelafter steel of FIG. 1A is tensioned. As such, it can be confirmed that,in a case in which the austenite grain is coarsened to generate thenonideal coarse grain in the microstructure of steel, surface qualitiesare degraded after a process thereof as described in FIG. 1B. However,in FIG. 2, illustrating an image of the microstructure of InventiveExamples, uniform grains without a nonideal coarse austenite grain isformed, thereby generating excellent surface processing qualities evenafter the process thereof.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentinvention as defined by the appended claims.

1. Steel for low temperature environments having excellent surfaceprocessing qualities, comprising: 15 wt % to 35 wt % of manganese (Mn),carbon (C) satisfying 23.6C+Mn≧28 and 33.5C—Mn≦23, 5 wt % or lower ofcopper (Cu) (excluding 0 wt %), chrome (Cr) satisfying 28.5C+4.4Cr≦57(excluding 0 wt %), 0.01 wt % to 0.5 wt % of titanium (Ti), 0.003 wt %to 0.2 wt % of nitrogen (N), iron (Fe) as a residual component, andinevitable impurities, wherein Ti and N satisfy Relational Formula 1below,1.0≦Ti/N≦4.5,  [Relational Formula 1] where Mn, C, Cr, Ti, and N in eachexpression refer to wt % of a content of each component.
 2. The steelfor low temperature environments having excellent surface processingqualities of claim 1, wherein the steel comprises a TiN precipitatehaving a size of 0.01 μm to 0.3 μm.
 3. The steel for low temperatureenvironments having excellent surface processing qualities of claim 1,wherein the steel comprises a TiN precipitate in an amount of 1.0×10⁷ to1.0×10¹⁰ per 1=².
 4. The steel for low temperature environments havingexcellent surface processing qualities of claim 1, wherein a number ofaustenite grains having a size of 200 μm or greater is 5 or less per 1cm² in a microstructure of the steel.
 5. The steel for low temperatureenvironments having excellent surface processing qualities of claim 1,wherein a microstructure of the steel comprises austenite in an areafraction of 95% or greater.
 6. The steel for low temperatureenvironments having excellent surface processing qualities of claim 5,wherein a carbide present in a grain boundary of austenite is lower thanor equal to 5% in an area fraction.
 7. The steel for low temperatureenvironments having excellent surface processing qualities of claim 1,wherein impact toughness of the steel is higher than or equal to 41 J ata temperature of −196° C.
 8. A method of manufacturing steel for lowtemperature environments having excellent surface processing qualities,comprising: providing a slab including 15 wt % to 35 wt % of Mn, Csatisfying 23.6C+Mn≧28 and 33.5C—Mn≦23, 5 wt % or lower of Cu (excluding0 wt %), Cr satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %), 0.01 wt % to0.5 wt % of Ti, 0.003 wt % to 0.2 wt % of N, Fe as a residual component,and inevitable impurities, Ti and N satisfying Relational Formula 1below; heating the slab at a temperature of 1050° C. to 1250° C.; andmanufacturing heat-rolled steel by heat rolling the slab that has beenheated,1.0≦Ti/N≦4.5,  [Relational Formula 1] where Mn, C, Cr, Ti, and N in eachexpression refer to wt % of a content of each component.
 9. The methodof claim 8, wherein the steel comprises a TiN precipitate having a sizeof 0.01 μm to 0.3 μm.
 10. The method of claim 8, wherein the steelcomprises a TiN precipitate in an amount of 1.0×10⁷ to 1.0×10¹⁰ per 1mm².
 11. The method of claim 8, wherein a number of austenite grainshaving a size of 200 μm or greater is 5 or less per 1 cm² in amicrostructure of the steel.